Dental porcelains

ABSTRACT

Opaque porcelains for use with metal cores in the manufacture of PFM restorations. The porcelains exhibit a coefficient of thermal expansion (CTE) substantially equal to or slightly above the CTE of the metal to which it is applied. The porcelains exhibit a CTE equal to or up to about 1.5×10 −6 /° C. higher than the dental alloys to which they are applied as the opaque. The porcelains are fabricated from a mixture of two frit compositions. A high expansion, leucite containing frit is combined with a low melting glass frit to provide a porcelain having an expansion in the range of 16.9 to about 18×10 −6 /° C. in the temperature range of 25°-500° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional application Ser. No.60/142,204 filed Jul. 2, 1999 entitled Dental Porcelains For Use InAll-Ceramic And Porcelain-Fused-To Metal Restorations which is herebyincorporated by reference.

FIELD OF THE INVENTION

This invention relates to low fusing high-expansion dental porcelainespecially useful for the fabrication of both all-ceramic andporcelain-fused-to-metal (PFM) restorations.

BACKGROUND OF THE INVENTION

Porcelains are typically designed to be used in the manufacture ofeither all-ceramic dental restorations or in PFM restorations, but arenot normally functional with both types of restorations due to thedifferences in properties of ceramics and metals. One such porcelain,OPC® Low Wear™ porcelain, available from Jeneric/Pentron Inc.,Wallingford, Conn. and covered in copending, commonly assigned patentapplication Ser. No. 09/133,582 filed Aug. 13, 1998, now U.S. Pat. No.6,120,591, which is hereby incorporated by reference, was initiallydesigned to be used as overlay for pressed all-ceramic restorations aswell as for the fabrication of porcelain jacket crowns and veneers.However, OPC®Low Wear™ porcelain is not currently used for PFMrestorations despite its wear resistance, forgiveness to naturaldentition and strength being superior to those of conventional PFMporcelains as shown in the Table 1 below:

TABLE 1 OPC ® Low Wear ™ Conventional Porcelain Property Porcelain forPFM Leucite average grain About 2-3 About 5-8 Size, μm Leucite volumefraction, % 35-40 20-25 Enamel wear*, × 10⁻² mm² 7.69 ± 3.20 18.23 ±5.20  Wear of ceramics*, × 10⁻² 0.16 ± 0.04 0.49 ± 0.11 mm³ CTE, 10⁻⁶/°C., 25° C.- About 17 About 13 500° C.

The major obstacle preventing use of the OPC® Low Wear™ porcelain in PFMrestorations is the absence of an opaque/alloy combination compatiblewith this porcelain having relatively high expansion of about 17×10⁻⁶/°C. (25° C.-500° C.).

There exists a Golden Gate System™ for PFM restorations availablethrough Degussa™ (Dental Division, South Plainfield, N.J.) whichcombines Duceragold™ porcelain and Degunorn™, type IV crown and bridgealloy (CTE=16.4×10⁻⁶/° C., 25° C.-500° C.). This system requires rathertedious multistep alloy preparation procedures including a necessarywash bake step prior to application of the opaque; and an excessivelylong (16-20 min) and complex first dentine bake to assure proper bondingand compatibility of the Duceragold™ porcelain to the Degunorm™ alloy.In particular, the cooling segment (3-4 min between 720° C. and 680° C.)in the first dentine bake is required by the manufacturer to growadditional leucite and may be an indication of instability of leucite inthis porcelain. The following Table 2 below sets forth the variousproperties of the Duceragold™ porcelain.

TABLE 2 Duceragold ™ Firing Temperature, ° C. 770-790 Glass Transition490 Temperature, ° C. Softening Temperature, ° C. 595 CTE 25°-500° C.,10⁻⁶/° C. 15.8 Recommended alloy Degunorm Alloy CTE 25° C.-500° C., 16.410⁻⁶/° C.

U.S. Pat. No. 5,453,290 to Van der Zel is directed to a dental porcelainfor use with a dental alloy. The porcelain described therein must befabricated from three frits, making it more difficult and costly tocontrol the expansion and the glass transition temperature of the finalproduct. Moreover, the CTE of the porcelain must be below the CTE of thealloy by 0.5-1.5 limiting the components to be used together. There is aneed to provide a porcelain-fused-to-metal system for dentalrestorations having simple manufacturing procedures. It is desirable toprovide a porcelain that is compatible with alloys of relatively highexpansion. It is desireable to provide a two-frit porcelain for use inPFM restorations.

SUMMARY OF THE INVENTION

These and other objects and advantages are accomplished by opaqueporcelains for use with metal cores in the manufacture of PFMrestorations. The porcelains exhibit a coefficient of thermal expansion(CTE) substantially equal to or slightly above the CTE of the metal towhich it is applied. In a preferred embodiment, the porcelains exhibit aCTE equal to or up to about 1.0×10⁻⁶/° C. higher than the dental alloysto which they are applied as the opaque. The porcelains are fabricatedfrom a mixture of two frit compositions. A high expansion, leucitecontaining frit is combined with a low fusing glass frit to provide aporcelain having an expansion in the range of 16.9 to about 18.5×10⁻⁶/°C. in the temperature range of 25°-500° C. By combining two frits, theexpansion and fusing temperature can be controlled to the values statedabove.

DESCRIPTION OF THE INVENTION

The invention relates to a porcelain material for use in all-ceramicrestorations and PFM restorations. The porcelains exhibit a coefficientof thermal expansion (CTE) substantially equal to or slightly above theCTE of the metal to which it is applied. Preferably, the porcelainsexhibit a CTE equal to or up to about 1.5×10⁻⁶/° C. higher than thedental alloys to which they are applied as the opaque, and morepreferably equal to or up to about 1.0×10⁻⁶/° C. The porcelains arecompatible with metals having coefficients of thermal expansion (CTE) inthe range of from about 15.5 to about 17×10⁻⁶/° C. in the temperaturerange from 20° to 500° C. The porcelains exhibit CTEs in the range ofabout 16.9 to about 18.5 in the temperature range from 20° to 500° C.,and preferably in the range of about 17 to about 17.5×10⁻⁶/° C. in thetemperature range from 20° to 500° C. The porcelains are fabricated froma mixture of two frit compositions. A high expansion, leucite containingfrit is combined with a low fusing glass frit to provide a porcelainhaving an expansion in the range of 16.9 to about 18.5×10⁻⁶/° C. in thetemperature range of 25°-500° C. It is essential to this invention thatboth the high expansion and the low fusing components of the two-fritmixture exhibit a low glass transition temperature (GTT). It isextremely important that the low fusing component used in the opaqueformulation exhibits a GTT lower than about 415° C. By combining twofrits, the expansion and firing temperature can be controlled to thevalues stated above. Opaque porcelains herein having pigments exhibit acoefficient of thermal expansion of average value of about 17×10⁻⁶/° C.in the temperature range of 25°-500° C. Opaque porcelains withoutpigments, i.e., white opaques, exhibit coefficients of thermal expansionin the higher end of the range, such as about 17.5 to about 18.5×10⁻⁶/°C. in the temperature range of 25°-500° C.

Table 3 below shows the compositional ranges of the porcelains for usein the invention.

TABLE 3 Body and Incisal Opaque Opaque Opaque Opaque Porcelain Porcelain(1) Porcelain (2) Porcelain (3) Porcelain (4) (by weight (by weight (byweight (by weight (by weight percent) percent) percent) percent)percent) SiO₂ about 59- about 59- about 59- about 48- about 48- about 65about 65 about 65 about 65 about 65 B₂O₃ X X X 0-about 0.7 0-about 0.7Al₂O₃ about 10- about 10- about 10- about 10- about 10- about 15 about15 about 15 about 15 about 15 ZnO X X X 0-about 5 0-about 5 CaO about0.5- about 0.5- about 0.5- about 0.5- about 0.5- about 2 about 2 about 2about 2 about 2 MgO X X X 0-about 2 0-about 2 BaO X X X 0-about 10-about 1 Li₂O about 1.5- about 1.5- about 1.5- about 1.5- about 1.5-about 3 about 3 about 3 about 3 about 3 K₂O about 15- about 15- about12- about 14- about 15- about 17 about 17 about 17 about 17 about 17Na₂O about 4- about 4- about 4- about 4- about 4- about 6 about 6 about6 about 6 about 6 TiO₂ X X X 0-about 2 0-about 2 ZrO₂ X X X 0-about 170-about 17 CeO₂ X X X 0-about 1 0-about 1 F about 0.4- about 0.4- about0.4- about 0.4- about 0.4- about 1 about 1 about 1 about 1 about 1 Ta₂O₅— X X 0-about 2 0-about 2 SnO2 — — — 0-about 18 0-about 18 ZrSiO₄ — — —0-about 7 — *Opacifiers 0-about 1 about 13- about 10- — — about 20 about20 **Pigments 0-about 5 about 2- — — — about 13 *Opacifiers, Al₂O₃,SnO₂, TiO₂, ZrO₂, ZrSiO₄, ZnO, CeO₂, or Ta₂O₅, are admixed as finepowder to a mixture of two frits. The resulting composition is referredto below as White Porcelain (opaque, body or incisal). **Pigments areadmixed as fine powder to a White Porcelain Powder. The resulting powdercomposition is referred to below as Shaded Porcelain (opaque, body orincisal). X signifies non-essential components.

As set forth in Table 3 above, Li₂O, present in an amount of from about1.5% to about 3%, and F, present in an amount of 0.4%-1%, areinstrumental in providing a low glass transition temperature. Thepresence of Li₂O and F also assist as well in increasing the coefficientof thermal expansion and decreasing the maturing (firing) temperature.The high expansion, leucite containing component of the two-frit mixturehas a reasonably low glass transition temperature as well. This isachieved by maintaining a reasonably low molar ratio of Al₂O₃ to the sumof alkali and alkaline earth oxides (R2O+RO). Normally, thesecompositions are extremely unstable and reactive as well as prone tosanidine precipitation in the temperature range of 650° C.-950° C. Itwas surprisingly found that certain compositions with specificcombinations of K₂O, Na₂O and Li₂O are very stable. In addition, it wasfound that the molar ratio of Al₂O₃/K₂O should be within the range of0.73-0.95 to assure both the required thermal stability and low glasstransition temperature of the high expansion component of the porcelain.The low glass transition temperature provides a porcelain having goodcompatability with alloys having CTEs in the range of about 15.5 toabout 17×10⁻⁶/° C. in the temperature range of 25°-500° C. Table 4 belowsets forth the properties of the porcelain compositions.

TABLE 4 PORCELAIN Body & Incisal Opaque (1) Opaque (3) Firing 855-870855-890 750-890 Temperature/Maturing Temperature, ° C. Glass Transition420-430 430-440 — Temperature, ° C. Softening 520 — — Temperature, ° C.CTE 25°-400° C., 15.2 — — 10⁻⁶/° C. CTE 25° - GTT, 15.8 — — 10⁻⁶/° C.(GTT = 430° C.) CTE 25°-500° C., 17.2 17.0 10⁻⁶/° C. CTE 25°-470° C., —— about 17- 10⁻⁶/° C. about 17.5

Table 5 sets forth compatible alloys for use with the porcelains.

TABLE 5 Alloy CTE (25° C.-500° C.) Application Bio-75G 15.5 ± 0.2 FastCool for single units only GoldCore 75 16.4 ± 0.2 For single units andbridges GoldCore 55 17.0 ± 0.2 For single units and bridges JewelCast17.0 ± 0.2 For single units and bridges

Table 6 below shows compositional examples of body (incisal) and opaqueporcelains.

TABLE 6 Example 2 Example 3 Example 4 Example 5 White White White WhiteOpaque Opaque Opaque Opaque Example 1 Porcelain Porcelain PorcelainPorcelain Body/Incisal Comparative for Light* for Light* for Dark** forDark** Porcelain Example 1 Shades Shades Shades Shades Two-frit MixtureComposition: SiO2 61.9 61.5 60.0 58.5 60.1 58.6 B2O3 0.5 0.5 0.6 0.6 0.60.6 Al2O3 11.6 15.4 13.6 13.6 13.7 13.6 ZnO 0.0 0.0 0.0 2.0 0.0 2.0 CaO1.7 0.6 1.2 1.2 1.2 1.2 MgO 0.8 0.2 0.2 0.1 0.2 0.1 BaO 0.0 0.0 0.4 0.40.4 0.4 Li2O 2.5 2.3 2.3 2.3 2.3 2.3 K2O 15.7 12.9 15.1 15.0 15.1 15.0Ta2O5 0.0 0.0 0.0 0.0 0.0 Na2O 4.8 6.0 5.5 5.4 5.4 5.3 TiO2 0.0 0.0 0.00.0 0.0 0.0 ZrO2 0.0 0.0 0.0 0.0 0.0 0.0 CeO2 0.0 0.6 0.3 0.3 0.3 0.3 F0.6 0.7 0.6 0.7 0.6 Mixed-in Opacifiers: ZrO2 17 17 15 15 ZrSiO4 TiO2SnO2 ZnO CeO2 Ta2O5 Firing 857 871 871 871 871 temperature CTE (25°C.-500° C.) 17.2 ± 0.3 17.8 17.8 17.8 17.8 *Light Opaque Shades -Pigment content < 6 wt % **Dark Opaque Shades - Pigment content > 6 wt %

The low glass transition temperature of the opaque porcelain isparamount to assure its compatibility with alloys having CTE's in therange of about 15.5 to about 17, such as commercially available GoldCore 75™ alloy from Jeneric/Pentron Inc., Wallingford, Conn. This is agold alloy that does not contain copper and other elements that formdark oxide layers and, therefore, requires much less intricatepreparation procedures compared to the Degunorm alloy. Specifically, theGold Core 75™ alloy forms an adequate oxide layer without compromisingthe appearance of the coping when degassed at 870° C.-885° C. for about5-7 min in air or vacuum.

The high potassium content in the porcelain is essential to assure highstability of leucite. High potassium oxide content combinedsynergetically with other alkali elements (Li and Na) assures as wellrelatively low glass transition temperature (GTT) and, hence, increasedresistance to thermal expansion mismatch cracking and increasedadaptability to alloys of slightly lower expansion. It was surprisinglyfound that increased potassium content increases stability incompositions with low GTT, e.g., Example 1 was found to be much morestable than Comparative Example 1 (compare K₂O content). Specifically,dental porcelain of Example 1 has excellent thermal expansion stabilityand maintains the same thermal expansion after 5 successive bakes at itsfiring temperature. Dental porcelain of Comparative Example 1 was foundto change thermal expansion and opacity upon multiple bakes.

Essential to this invention is that opaque compositions possess arelatively low transition temperature and contain the same elements suchas ZnO and Ta₂O₅ as the oxide layer forming on the alloy that assuregood bonding to alloys. Specifically, the oxide layer on the Gold Core75™ alloy was found to be enriched with ZnO and Ta₂O₅ and the samecomponents were included in the opaque formulation to improve bonding.

Body and incisal porcelain are typically applied to opaque porcelain,respectively. Preferably, the body and incisal porcelains used with theopaque herein exhibit an average coefficient of thermal expansion ofabout 17.2.

In a preferred embodiment of the invention, alloys having a CTE in therange of about 15.5 to about 17×10⁻⁶/° C. in the temperature range of25°-500° C. are used to manufacture a metal core for a restoration.Opaque porcelains are applied thereto, wherein the CTE is in the rangeof about 16.9 to about 17.5×10⁻⁶/° C. in the temperature range of25°-500° C. and body porcelains are applied thereto having CTEs in therange of about 16.9 to about 17.7. It is preferred that the opaqueporcelain has a CTE about equal to or up to about 1.5×10⁻⁶/° C. higherthan the metal core. It is preferred that the body porcelain has a CTEabout equal to or up to about 1.5×10⁻⁶/° C. higher than the metal core.It is preferable that the CTE of the opaque is between the CTE of thealloy and the CTE of the body porcelain.

The following examples illustrate the invention.

EXAMPLES

Copings and bridge frameworks cast from Bio-75G, GoldCore75, GoldCore55and JewelCast alloys available from Jeneric/Pentron, Wallingford, Conn.,were prepared with a carbide tool, sand-blasted with alumina sand atpressure of 2 bar and ultrasonically cleaned in water for about 5 min.The same degassing cycle given in the firing charts below was used forBio-75G, JewelCast, GoldCore55 and GoldCore75 castings. Followingdegassing, the oxide layer was removed by sand-blasting and castingswere ultrasonically cleaned in water for about 5 min. The opaque of thecomposition of Example 3 (Table 6) was applied in two thin coats andfired according to the firing cycle given in a table below. Body/Incisalporcelain of composition of Example 1 was used to build full contourcrowns and bridges and fired up to 5 times as per firing chart below.

No cracking was observed on single unit restorations made from thealloys above. However, cracks in pontic areas were found when porcelainwas fired onto bridge frameworks made from Bio-75G. Both single andmultiunit restorations made from GoldCore 75, GoldCore 55 and JewelCastexhibited no cracking upon multiple firings.

1^(st) 2^(nd) Opaque OPC OPC bake Low Low 3^(rd)-5^(th) OPC Degassing (2Wear Wear Low Wear cycle coats) bake bake bake Firing chart in ° F.Predry, min 0 6 6 6 6 Low T, ° F. 1200 600 1000 1000 1000 High T, ° F.1625 1600 1575 1550 1550 Rate, ° C./min 100 75 75 75 75 Vacuum 100% 100%100% 100% 100% VacOn,° F. 1200 750 1000 1000 1000 VacOff, ° F. 1625 15001525 1500 1500 Hold, min 5 in vacuum 0 0 0 0 Cool, min 0 0 0 0 0 Firingchart in ° C. Predry, min 0 6 6 6 6 Low T, ° C. 650 316 538 538 538 HighT, ° C. 885 871 857 843 843 Rate, ° C./min 55 42 42 42 42 Vacuum 100%100% 100% 100% 100% VacOn, ° C. 650 399 538 538 538 VacOff, ° C. 885 816829 816 816 Hold, min 5 in vacuum 0 0 0 0 Cool, min 0 0 0 0 0

In addition to dental restorations, bond flags were cast from the alloyslisted above. Two thin coats of opaque (composition of Example 3) wereapplied and fired onto the bond flags. Bond flags were bent using pliersand metal surface exposed along the bend where opaque is fractured wasinspected using optical stereomicroscope under magnification of 10×.Fracture along opaque-metal interface was found mostly adhesive, i.e.substantial fraction of the metal surface was covered by opaqueindicating good bonding between alloys and opaque. The observed coveragewas comparable to other metal-porcelain systems and therefore deemedsufficient. Bond strength was quantified according to ISO-9693Metal-Ceramic Bond Test (Schwickerath crack initiation test). Thefollowing Table indicates the bond strengths calculated from the formulaτ_(b)=k·F_(fail)

wherein τ_(b) is the debonding/crack initiation strength k is acoefficient which is a function of the thickness of the metal substrate,and the value of Young's modulus of the used metallic material; andF_(fail) is the fracture force

Elas- tic Thick- Mod- ness ulus Load F(fail) τb Specimen (mm) (GPa)(Lbs) Newtons K (MPa) gold core 75 0.55 12.5 2.81 12.49950291 3.644.99821048 gold core 75 0.54 12.5 2.262 10.06187743 3.75 37.73204037gold core 75 0.55 12.5 1.81 8.051281235 3.65 29.38717651 gold core 750.55 12.5 3.02 13.43362946 3.65 49.03274754 gold core 75 0.55 12.5 2.7812.36605626 3.65 45.13610535 mean 0 41.25725605 Std Dev 0 7.791412161gold core 55 0.5 15.12 1.42 6.316474781 4.1 25.8975466 gold core 55 0.515.12 1.07 4.759597194 4.1 19.5143485 gold core 55 0.52 15.12 2.8812.81087843 3.8 48.68133803 gold core 55 0.5 15.12 1.68 7.473012417 4.130.63935091 gold core 55 0.47 15.12 1.51 6.716814732 4.7 31.56902924gold core 55 0.47 15.12 0.98 4.359257243 4.7 20.48850904 mean 029.46502039 Std dev 9.724481694

As will be appreciated, the present invention provides porcelaincompositions compatible with alloys for use in the manufacture of PFMrestorations. The porcelains exhibit a coefficient of thermal expansion(CTE) substantially equal to or slightly above the CTE of the metal towhich it is applied. The porcelains are fabricated from a mixture of twofrit compositions. A high expansion, leucite containing frit is combinedwith a low melting glass frit to provide a porcelain having an expansionin the range of 16.9 to about 18×10⁶/° C. in the temperature range of25°-500° C. By combining two frits, the expansion and firing temperaturecan be easily controlled.

While various descriptions of the present invention are described above,it should be understood that the various features can be used singly orin any combination thereof. Therefore, this invention is not to belimited to only the specifically preferred embodiments depicted herein.

Further, it should be understood that variations and modificationswithin the spirit and scope of the invention may occur to those skilledin the art to which the invention pertains. Accordingly, all expedientmodifications readily attainable by one versed in the art from thedisclosure set forth herein that are within the scope and spirit of thepresent invention are to be included as further embodiments of thepresent invention. The scope of the present invention is accordinglydefined as set forth in the appended claims.

What is claimed is:
 1. A porcelain composition for use as an opaque ondental alloys in the manufacture of a dental restoration comprising byweight percent: about 48 to about 65% SiO₂; about 10 to about 15% Al₂O₃;about 0.5 to about 2% CaO; about 1.5 to about 3% Li₂O; about 15 to about17% K₂O; about 4 to about 6% Na₂O; and about 0.4 to about 1 F.
 2. Theporcelain of claim 1 possessing a coefficient of thermal expansionslightly higher than the dental alloys to which it is applied as theopaque.
 3. The porcelain of claim 1 wherein the coefficient of thermalexpansion is equal to or up to about 1.5×10⁻⁶/° C. higher than thedental alloys to which it is applied as the opaque.
 4. The porcelaincomposition of claim 1 possessing a coefficient of thermal expansionwhich is compatible with alloys possessing a coefficient of thermalexpansion in the range of about 15.5 to about 17×10⁻⁶/° C. in thetemperature range of 25°-500° C.
 5. The porcelain of claim 1 possessinga coefficient of thermal expansion of about 16.9 to about 18.5 in thetemperature range of 25° C. to 500° C.
 6. The composition of claim 1further comprising by weight: about 0 to about 0.7% B₂O₃; about 0 toabout 5% ZnO; about 0 to about 2% MgO; about 0 to about 1% BaO; about 0to about 2% TiO₂; about 0 to about 17% ZrO₂; about 0 to about 7% ZrSiO₄;about 0 to about 1% CeO₂; about 0 to about 2% Ta₂O₅; and about 0 toabout 18% SnO₂.
 7. A method of making a dental restoration comprising:forming a dental porcelain powder from a dental composition comprisingabout 48 to about 65% SiO₂, about 10 to about 15% Al₂O₃, about 0.5 toabout 2% CaO, about 1.5 to about 3% Li₂O, about 14 to about 17% K₂O,about 4 to about 6% Na₂O, and about 0.4 to about 1 F, wherein thematuring temperature is in the range of about 750° C. to about 890° C.;shaping the dental porcelain powder onto a metal core; and heating theshaped dental porcelain powder to between about 750° C. to about 880° C.to fuse the dental porcelain powder to the metal core; wherein the metalcore exhibits a coefficient of thermal expansion in the range from about15.5 to about 17×10⁻⁶/° C. (measured from 25° C. to 500° C.).
 8. Themethod of claim 7 wherein the porcelain has a coefficient of thermal inthe range of about 17 to about 17.5 in the temperature range of 25° C.to 470° C.
 9. The method of claim 7 wherein the metal frameworkcomprises a gold alloy.
 10. The method of claim 7 wherein the porcelainpowder is an opaque porcelain.
 11. The method of claim 10 furthercomprising applying a body porcelain over the opaque porcelain.
 12. Themethod of claim 11 further comprising applying an incisal porcelain overthe body porcelain.
 13. The method of claim 12 wherein the incisalporcelain comprises: about 59 to about 65% SiO₂; about 10 to about 15%Al₂O₃; about 0.5 to about 2% CaO; about 1.5 to about 3% Li₂O; about 15to about 17% K₂O; about 4 to about 6% Na₂O; and about 0.4 to about 1 F.14. The method of claim 12 wherein the body porcelain comprises: about59 to about 65% SiO₂; about 10 to about 15% Al₂O₃; about 0.5 to about 2%CaO; about 1.5 to about 3% Li₂O; about 15 to about 17% K₂O; about 4 toabout 6% Na₂O; and about 0.4 to about 1 F.
 15. The method of claim 13wherein the incisal porcelain comprises a mixture of a high expansionleucite-containing frit and a low fusing glass frit.
 16. A porcelaincomposition comprising by weight percent: about 59 to about 65% SiO₂;about 10 to about 15% Al₂O₃; about 0.5 to about 2% CaO; about 1.5 toabout 3% Li₂O; about 12 to about 17% K₂O; about 4 to about 6% Na₂O;about 0.4 to about 1% F; and about 10 to about 20% of an opacifier;wherein the porcelain composition is used as an opaque in themanufacture of dental restorations.
 17. The porcelain composition ofclaim 16 wherein the opacifier is selected from Al₂O₃, ZnO, TiO₂, ZrO₂,ZrSiO₄, CeO₂, Ta₂O₅, SnO₂ and mixtures thereof.
 18. A dental restorationcomprising: a metal core, having a coefficient of thermal expansionbelow about 17×10⁻⁶/° C. (measured from 25° C. to 500° C.); and anopaque porcelain applied on the metal core, having a coefficient ofthermal expansion about equal to or up to about 1.5×10⁶/° C. higher thanthe metal core; wherein the opaque porcelain comprises about 48 to about65% SiO₂, about 10 to about 15% Al₂O₃; about 0.5 to about 2% CaO; about1.5 to about 3% Li₂O; about 15 to about 17% K₂O; about 4 to about 6%Na₂O; and about 0.4 to about 1 F.
 19. The dental restoration of claim 18wherein the opaque porcelain comprises by weight: about 48 to about 65%SiO₂; about 10 to about 15% Al₂O₃; about 0.5 to about 2% CaO; about 1.5to about 3% Li₂O; about 14 to about 17% K₂O; about 4 to about 6% Na₂O;and about 0.4 to about 1 F.
 20. The dental restoration of claim 19wherein the opaque porcelain further comprises: about 0 to about 0.7%B₂O₃; about 0 to about 5% ZnO; about 0 to about 2% MgO; about 0 to about1% BaO; about 0 to about 2% TiO₂; about 0 to about 17% ZrO₂; about 0 toabout 7% ZrSiO_(4;) about 0 to about 1% CeO₂; about 0 to about 2% Ta₂O₅;and about 0 to about 18% SnO₂.
 21. A dental restoration comprising: ametal core, having a coefficient of thermal expansion below about17×10⁻⁶/° C. (measured from 25° C. to 500° C.); an opaque porcelainapplied on the metal core, having a coefficient of thermal expansionabout equal to or up to about 1.5×10⁻⁶/° C. higher than the metal core;and a body porcelain applied to the opaque porcelain having acoefficient of thermal expansion about equal to or up to about1.5×10⁻⁶/° C. higher than the metal core.
 22. The dental restoration ofclaim 21 wherein the coefficient of thermal expansion of the opaqueporcelain is between the coefficient of thermal expansion of the alloyand the coefficient of thermal expansion of the body porcelain.
 23. Aporcelain composition made from a mixture of a high expansionleucite-containing frit and a low fusing glass frit comprising: about 59to about 65% SiO₂; about 10 to about 15% Al₂O₃; about 0.5 to about 2%CaO; about 1.5 to about 3% Li₂O; about 15 to about 17% K₂O; about 4 toabout 6% Na₂O; and about 0.4 to about 1 F.
 24. The porcelain compositionof claim 23 further comprising up to 1% opacifiers and up to 5%pigments.
 25. The porcelain composition of claim 23 further comprising13 to 20% opacifiers and 2 to 13% pigments.
 26. The porcelaincomposition of claim 23 wherein the ratio of Al₂O₃:K₂O is within therange of 0.73 to 0.95.